Process for preparing N6 substituted aminopurine ribofuranose nucleosides

ABSTRACT

An improved process for preparing N6-substituted aminopurine ribofuranose nucleosides. Compounds of this type are known to be usefull in the prepartation of compounds having activitity at adenosine receptors, e.g. Adenosine A1 receptor. The process comprises the step of reacting a 6-halopurine ribofuranose nucleoside with an amine in the presence of CaCO3, wherein acid is added to the reaction mixture.

FIELD OF THE INVENTION

The present invention relates to an improved process for the preparationof N⁶-substituted aminopurine ribofuranose nucleosides. Moreparticularly, the invention is concerned with an improved process forpreparing N⁶-substituted aminopurine ribofuranose nucleosides byamination of the corresponding 6-halopurine ribofuranose nucleoside inthe presence of CaCO₃. Compounds of this type are known to be useful inthe preparation of compounds having activity at adenosine receptors,e.g. the Adenosine A1 receptor.

BACKGROUND OF THE INVENTION

Adenosine A1 agonists and processes for their preparation are describedin: EP0322242, WO97/43300, WO99/24449, WO99/24450, WO99/24452,WO99/67262, WO98/16539 (Novo Nordisk A/S); WO98/04126 (Rhone-PoulencRorer Pharmaceuticals Inc.); and WO98/01459 (Novo Nordisk A/S). Forexample, in WO99/67262 (Glaxo Group Limited), adenosine derivatives areprepared by reaction of a halopurine ribofuranose nucleoside with anamine either in the absence or presence of a solvent such as an alcohol,an ether, a substituted amide, a halogenated hydrocarbon, an aromatichydrocarbon, dimethyl sulfoxide (DMSO) or acetonitrile, preferably at anelevated temperature, in the presence of a suitable acid scavenger, forexample, inorganic bases such as sodium, cesium or potassium carbonate,or organic bases such as triethylamine, diisopropylethylamine orpyridine, optionally in the presence of a palladium catalyst andphosphine ligand.

An alternative process for the preparation of arylamines is described inU.S. Pat. No. 5,576,460 (Buchwald). This process involves reacting anamine with an aryl halide compound in the presence of a nickel orpalladium catalyst. The present inventors found, however, that whenreacting 6-halopurine ribofuranose nucleosides with certain amines (e.g.4-chloro-2-fluoroaniline), the process had low reproducibility andtended to give incomplete reaction and very low yield. The incompletereaction, in addition to the necessity of removing the palladium heavymetal makes the process impractical for preparing certain 6-substitutedaminopurine ribofuranose nucleosides.

CaCO₃ processes for preparing adenosihe derivatives are described inKwatra et al., (1987) J. Med. Chem. 30:954, Fleysher et al., (1969) J.Med. Chem. 12:1056 and Yadava et al. (1988) Himalayan Chem. Pharm Bull.5:31. For example, Fleysher et al. describes the synthesis ofN⁶-phenyladenosine from 6-chloropurine riboside in absolute ethanol inthe presence of CaCO₃. However, the present inventors found that whenanilines of poor nucleophilicity like 4-chloro-2-fluoroaniline werereacted with 6-chloropurine riboside under these conditions, thereaction was slow and a significant amount of N⁶-aryladenine by-productwas derived from cleavage of the glycosyl linkage. The reaction was alsofound to be sensitive to the choice of solvents, for example, it had nomeaningful conversion in 2-methoxyethanol and provided only 25% yield ofproduct in isopropanol.

The problem to be solved by the present invention was therefore toprovide an improved process for the preparation of N⁶-substitutedaminopurine ribofuranose nucleosides.

SUMMARY OF THE INVENTION

The present inventors have surprisingly found that when a CaCO₃ processis used for preparing N⁶-substituted aminopurine ribofuranosenucleosides, the addition of acid at the start of the reactionaccelerates the reaction, improves the yield, and/or reduces thequantity of acid hydrolysis by-products generated. It is surprising thatthe addition of acid improves the efficiency of the reaction instead ofneutralising the base (CaCO₃) and preventing the reaction. Previously,CaCO₃ had been used as an acid scavenger (base) to promote reactions inwhich an acid was generated.

Accordingly, the present invention provides a process for preparingN⁶-substituted aminopurine ribofuranose nucleosides comprising the stepof reacting a 6-halopurine ribofuranose nucleoside (e.g. a6-chloropurine ribofuranose nucleoside) with an amine (e.g. an aliphaticamine, alicyclic amine or aromatic amine) in the presence of CaCO₃,characterised in that acid is added to the reaction mixture at the startof the reaction.

A further aspect of the invention is the use of the process of theinvention in the preparation of compounds which have activity atadenosine receptors, e.g. the Adenosine A1 receptor.

DETAILED DESCRIPTION OF THE INVENTION

A preferred aspect of the invention is the use of the process of theinvention in the preparation of N⁶-aminopurine ribofuranose nucleosidesas described in EP0322242, WO97/43300, WO99/24449, WO99/24450,WO99/24452, WO99/67262, WO98/16539 (Novo Nordisk A/S); WO98/04126(Rhone-Poulenc Rorer Pharmaceuticals Inc.); and WO98/01459 (Novo NordiskA/S) which are all incorporated herein by reference in their entirety.

Another preferred aspect of the invention is a process for preparing aN⁶-substituted aminopurine ribofuranose nucleoside of formula (I):

comprising the step of reacting a 6-halopurine ribofuranose nucleosideof formula (II) with an amine of formula (III) in the presence of CaCO₃,characterised in that acid is added to the reaction mixture at the startof the reaction;wherein the 6-halopurine ribofuranose nucleoside of formula (II) and theamine of formula (III) are:

wherein L represents halogen;

-   R² represents C₁₋₃alkyl, C₁₋₃alkenyl, C₁₋₃alkoxy, halogen or    hydrogen;-   R³ represents (i) hydrogen, (ii) C₁₋₆alkyl optionally substituted by    one or more halogens, (iii) C₁₋₆ alkylOCH₂— where the alkyl chain is    optionally substituted by one or more halogens, (iv) an acetylene    group, or (v) a 5-membered heterocyclic group optionally substituted    by: C₁₋₆alkoxy-, —C₁₋₆alkylO(CH₂)_(n)— where n is 0-6,    C₃₋₇cycloalkyl, C₁₋₆hydroxyalkyl, halogen or a —C₁₋₆alkyl,    —C₁₋₆alkenyl or —C₁₋₆alkynyl group optionally substituted by one or    more halogens;-   R⁴ and R⁵ independently represent hydrogen, acyl, —C₁₋₆alkyl or a    suitable protecting group (e.g. acetyl or a protecting group wherein    R⁴ and R⁵ together form an alkylidene group);-   R¹ represents hydrogen or a group selected from:-   (i) -(alk)_(n)-(C₃₋₉)cycloalkyl or -(alk)_(n)-(C₃₋₉)cycloalkenyl,    said cycloalkyl or cycloalkenyl group being optionally substituted    by one or more substituents selected from OH, halogen, C₁₋₆alkyl,    —C₁₋₆alkoxy, C₂₋₆alkenyloxy-, C₂₋₆ alkynyloxy-, and phenyl, wherein    (alk) represents C₁₋₃alkyl and n represents 0 or 1, and said (alk)    group may be optionally substituted by a C₃₋₆cycloalkyl group;    -   (ii) a phenyl group optionally substituted by one or more        substituents selected from: halogen, OH, CF₃, cyano, —C₁₋₆alkyl,        —C₂₋₆alkenyl, —C₂₋₆alkynyl, C₁₋₆alkoxy-, —C₁₋₆alkylOH, —CO₂H and        —CO₂C₁₋₆ alkyl;-   (iii) a C₄₋₇aliphatic heterocyclic group containing at least one    heteroatom selected from O, N or S, and optionally substituted by    one or more substituents selected from: OH, —C₁₋₆alkyl, —C₁₋₆alkoxy,    —CO₂(C₁₋₄)alkyl, —CO(C₁₋₄)alkyl, —CO₂aryl or    —CO₂(alk)_(n)(C₃₋₆)cycloalkyl, wherein (alk) represents C₁₋₃alkyl    and n represents 0 or 1;    -   (iv) a straight or branched C₁₋₁₂alkyl group optionally        substituted by one or more groups selected from phenyl, halogen,        hydroxy, and C₃₋₇ cycloalkyl, wherein one or more carbon atoms        of the C₁₋₁₂alkyl group may be optionally replaced by a group        independently selected from S(═O)_(n) (where n is 0, 1 or 2) and        N;    -   (v) a fused bicyclic ring        wherein A represents C₄₋₆ cycloalkyl or phenyl and B represents        phenyl optionally substituted by C₁₋₃alkyl, and the bicyclic        ring is attached to the purine-6-amino moiety via a ring atom of        ring A.

In an alternative aspect R¹ represents:

-   (i) -(alk)_(n)-(C₃₋₉)cycloalkyl, including bridged cycloalkyl,    optionally substituted by one or more substituents selected from:    OH, halogen, —C₁₋₃alkoxy, or phenyl wherein (alk) represents    C₁₋₃alkyl or C₁₋₃alkylene and n represents 0 or 1;-   (ii) a C₄₋₇aliphatic heterocyclic group containing at least one    heteroatom selected from O, N or S, and optionally substituted by    one or more subsituents selected from: OH, —C₁₋₆alkyl, —C₁₋₆alkoxy    —CO₂(C₁₋₄)alkyl, and —CO(C₁₋₃alkyl);-   (iii) a straight or branched C₁₋₁₂ alkyl, optionally including one    or more O, S(═O)_(n) (where n is 0, 1 or 2) and N groups substituted    within the alkyl chain, said alkyl optionally substituted by one or    more of the following groups: phenyl, halogen, hydroxy or    C₃₋₇cycloalkyl;-   (iv) a phenyl group optionally substituted by one or more    substituents selected from: halogen, CF₃, cyano, —C₁₋₆ alkyl, —C₂₋₆    alkenyl, —C₁₋₆alkoxy, —C₁₋₆ alkylOH, —CO₂H and —CO₂C₁₋₆ alkyl.

As used herein, the terms “alkyl” and “alkoxy” mean both straight andbranched chain saturated hydrocarbon groups. Examples of alkyl groupsinclude methyl, ethyl, propyl and butyl groups. Examples of alkoxygroups include methoxy and ethoxy groups. Other examples include propoxyand butoxy. The term “hydroxyalkyl” means both straight and branchedchain saturated hydrocarbon groups substituted by a hydroxy group. Alkylgroups may be unsubstituted, or substituted with one to foursubstituents, preferably one to three substituents as definedhereinabove.

One to three, preferably one or two, carbon atoms of an alkyl chain maybe replaced by a group independently selected from S(═O)_(n) (where n is0, 1 or 2) and N. When the heteroatom N replaces a carbon atom in aC₁₋₁₂alkyl group the N atom will, where appropriate be substituted byone or two substituents selected from hydrogen and C₁₋₆alkyl.

As used herein, the terms “alkenyl”, “alkynyl”, “alkenyloxy” and“alkynyloxy” mean both straight and branched chain unsaturatedhydrocarbon groups. Examples of alkenyl groups include ethylene andpropylene. Examples of alkynyl groups include ethynyl and propynyl.Examples of alkynyloxy groups include propynyloxy and ethynyloxy.Examples of alkenyloxy groups include propenyloxy and ethenyloxy.

As used herein, the term “halo” or ‘halogen’ means fluorine, chlorine,bromine or iodine.

As used herein, the term “acyl” means a straight or branchedC₁₋₆alkyl-C═O group.

As used herein the term “aryl” means monocyclic or bicyclic aromaticcarbocyclic groups such as phenyl or naphthyl, preferably phenyl.

As used herein, the term “cycloalkyl” means an aliphatic grouppreferably having 3 to 9 carbon atoms in the ring system. The cycloalkylgroup can be monocyclic or bicyclic. A bicyclic group may be fused orbridged. Preferably, the cycloalkyl group is monocyclic. Examples ofmonocyclic cycloalkyl groups include cyclopropyl, cyclobutyl,cyclopentyl, cyclohexyl and cycloheptyl. Another example of a monocycliccycloalkyl group is cyclooctyl. Examples of bicyclic cycloalkyl groupsinclude bicyclo[2.2.1]hept-2-yl. Cycloalkyl groups may be unsubstituted,or substituted with one to four substituents, preferably one or twosubstituents as defined hereinabove.

As used herein, the term “cycloalkenyl” means a partially unsaturatedaliphatic group having 3 to 9 carbon atoms in the ring system. Thecycloalkenyl group can be monocyclic or bicyclic. Preferably, thecycloalkyl group is monocyclic. Examples of monocyclic cycloalkenylgroups include cyclopentenyl and cyclohexenyl. Cycloalkenyl groups maybe unsubstituted, or substituted with one to four substituents,preferably one or two substituents as defined hereinabove.

As used herein, the term “heterocyclic group” means rings containing oneor more heteroatoms selected from: nitrogen, sulphur and oxygen. Theheterocycle may be aromatic or non-aromatic, i.e., may be saturated,partially or fully unsaturated. Examples of 5-membered groups includeisoxazole, oxadiazole, pyrazole, oxazole, triazole, tetrazole andthiadiazole.

When the heteroatom N replaces a carbon atom in a C₁₋₁₂alkyl group the Natom will, where appropriate be substituted by one or two substituentsselected from hydrogen and C₁₋₆alkyl.

As used herein, the term “aliphatic heterocyclic group” as defined forR¹ means a cyclic group of 4 to 7 carbon atoms wherein one or more ofthe carbon atoms is/are replaced by heteroatoms independently selectedfrom nitrogen, oxygen or sulfur. This group may be unsubstituted, orsubstituted with one to four substituents, preferably one or twosubstituents as defined hereinabove. Examples of aliphatic heterocyclicgroups include piperidinyl, tetrahydrofuranyl and tetrahydropyranyl.

As used herein, the term “aliphatic amine” means an amine groupcomprising straight or branched chains of carbon atoms, saturated orunsaturated.

As used herein, the term “alicyclic amine” means an amine groupcomprising at least one closed ring of carbon atoms, e.g. cycloalkylgroups.

As used herein, the term “aromatic amine” means an amine groupcomprising at least one benzene ring.

Preferably, L represents chlorine.

Preferably, R² represents halogen or hydrogen, more preferably hydrogen.

Preferably, R³ represents an acetylene group, or a 5-memberedheterocyclic group optionally substituted by a C₁₋₄alkyl. Preferably theheterocyclic group is selected from an isoxazole, oxadiazole, pyrazole,oxazole, triazole, tetrazole or thiadiazole, more preferably anisoxazole, a 1,2,4- or 1,3,4-oxadiazole.

Preferably R⁴ and R⁵ represent hydrogen, or together form an alkylidenegroup. More preferably, R⁴ and R⁵ together form an alkylidene group.

Conveniently, R¹ may represent (alk)_(n)-C₃₋₉cycloalkyl wherein n is 0or 1 and the said cycloalkyl is either unsubstituted or substituted byat least one substituent selected from halogen, particularly fluorine,and OH. Alternatively the cycloalkyl group may be either unsubstitutedor substituted by at least one substituent selected from —C₁₋₆alkyl,C₁₋₆alkoxy-, phenyl and OH. Further alternative substituents include atleast one substituent selected from halogen, C₂₋₆alkenyloxy-, and —C₃₋₆cycloalkyl. More preferably, the cycloalkyl group is unsubstituted ormonosubstituted with OH or C₁₋₃ alkyl, yet more preferably by OH. Thecycloalkyl group may also be monosubstituted by C₂₋₆alkenyloxy- or—C₃₋₆cycloalkyl, or substituted by one or two halogen atoms. Preferablythe cycloalkyl ring has 3 to 8 carbon atoms, more preferably 5 or 6carbon atoms. Cycloalkyl groups include hydroxycyclopentyl ormethoxycyclohexyl. Other cycloalkyl groups includepropenyloxycyclohexyl, ethyloxycyclohexyl, difluorocyclohexyl,dicyclopropylmethyl, cyclooctyl and cycloheptyl. Preferably n is zero.When n is 1 and the (alk) group is substituted, substituents includecyclopropyl.

R¹ may represent (alk)_(n)-(C₃₋₉)cycloalkenyl wherein n is 0 or 1 andthe said cycloalkenyl is unsubstituted or substituted by at least onesubstituent selected from —C₁₋₆alkyl, C₁₋₆alkoxy-, phenyl and OH.Alternative substituents include at least one substituent selected fromhalogen, —C₂₋₆ alkenyloxy, and —C₃₋₆cycloalkyl. Preferably n is zero.More preferably, the cycloalkenyl group is unsubstituted. Preferably thecycloalkenyl ring has 5 or 6 carbon atoms, more preferably the ring iscyclohexenyl.

Alternatively, R¹ may represent a substituted or unsubstituted aliphaticheterocyclic group, the substitutent being selected from C₁₋₆alkyl, or—CO₂(C₁₋₄)alkyl. The substituent may also be —CO₂phenyl or—CO₂(alk)_(n)(C₃₋₆)cycloalkyl. Preferably the aliphatic heterocyclicring is 6 membered and more preferably contains only one O, N or Sheteroatom. Conveniently, the aliphatic heterocyclic group isunsubstituted or, when substituted, the substituent is —CO₂(C₁₋₄)alkylor —CO₂(alk)_(n)(C₃₋₆)cycloalkyl or —CO₂phenyl, the heteroatom is N andthe substituent is directly attached to said ring nitrogen atom.Preferably when the heterocycle is substituted with —CO₂(C₁₋₄)alkyl, theheteroatom is N and the substituent is directly attached to said ringnitrogen atom. Most preferably when the heterocyclic ring isunsubstituted the heteroatom is O. Most preferably when the heterocyclicring is substituted the heteroatom is N.

Alternatively, R¹ may represent a straight or branched alkyl of 1-6carbon atoms optionally with at least one S(═O)_(n) and where S(═O)_(n)is present, optionally substituted with N at a position adjacent to theS(═O)_(n) group; where there is an group is preferred; where there is anS(═O)_(n) in the chain, preferably n is 1 or 2, more preferably n is 2.The alkyl group conveniently may be unsubstituted or substituted by atleast one OH group.

Alternatively R¹ may represent a phenyl group which is substituted byone or two substituents selected from OH, C₁₋₆alkyl, particularlyC₁₋₄alkyl and halogen. Preferably the phenyl is disubstituted in the 2,3 or 2, 4 or 2,5 positions. Preferably both substituents are halogenmore particularly, fluorine and chlorine. For example, a particularlypreferred combination is 2-fluoro and 4-chloro.

In an alternative aspect the phenyl is monosubstituted by C₁₋₆ alkyl,for example methyl.

In a preferred aspect of the invention, R¹ represents phenyl optionallysubstituted by halogen or C₁₋₆alkyl, -(alk)_(n)-C₃₋₆ cycloalkyloptionally substituted by OH, or a C₅₋₆ aliphatic heterocyclic groupcontaining one heteroatom selected from O, N or S and optionallysubsituted by —C₁₋₆alkyl or —CO₂C₁₋₄alkyl.

It is to be understood that the present invention covers allcombinations of particular and preferred groups mentioned above.

The process of the present invention involves the addition of acid tothe reaction mixture at the start of the reaction. The addition of acidto the reaction mixture at the start of the reaction catalyses thereaction. Suitable acids include aliphatic and aromatic carboxylicacids, aliphatic and aromatic sulfonic acids, halogen acids (e.g. HCl,HBr, HI) or mineral acids (e.g. phosphoric acid, sulphuric acid, nitricacid). The acid is preferably acetic acid, p-toluenesulfonic acid,hydrochloric acid or trifluoroacetic acid, more preferably acetic acidor hydrochloric acid, most preferably acetic acid. Carboxylic acids maybe used as a solvent for the amination of 6-halopurine nucleosides oradded into the reaction mixture as a separate ingredient (i.e. as acatalyst). When the acid is added as a separate ingredient, suitablesolvents include alcohols (MeOH, EtOH, propanol, isopropanol, t-butylalcohol), toluene, N,N-dimethylformamide (DMF), dimethyl sulfoxide(DMSO), ethers and MeCN. The acid is preferably present in the range5-20 mol of the reaction mixture.

The process of the invention is suitably carried out at elevatedtemperature. Preferably the temperature is in the range 50-120° C., morepreferably 80-120° C., most preferably 85-95° C.

Compounds of formula (II) may be prepared by any method known in theart. For example, a suitable method for the preparation of compounds offormula (II) is disclosed in WO99/67262. Compounds of formula (III) arewell known in the art.

The process of the invention can be used in the preparation of compoundswhich are active at adenosine receptor(s), by any method known in theart, for example as described in WO99/67262 EP0322242, WO97/43300,WO99/24449, WO99/24450, WO99/24452, WO99/67262, WO98/16539 (Novo NordiskA/S); WO98/04126 (Rhone-Poulenc Rorer Pharmaceuticals Inc.); WO98/01459(Novo Nordisk A/S) and as shown in the Examples. The compound may be anagonist or antagonist, but is preferably an agonist. It will beunderstood that the process of the invention can be used either in thepreparation of an intermediate or in the preparation of a finalcompound. A further aspect of the invention is therefore the use of theprocess of the invention in the preparation of a compound which isactive at one or more adenosine receptors, e.g. Adenosine A1 receptor,Adenosine A2a receptor. Another aspect of the invention is the use ofthe process of the invention in the preparation of an Adenosine A1agonist, e.g. (2S, 3S, 4R, 5R)-2-(5tert-butyl-[1,3,4]oxadiazol-2-yl)-5-[6-(4-chloro-2-fluoro-phenylamino)-purin-9-yl]-tetrahydro-furan-3,4-diol.

The following examples illustrate aspects of this invention but shouldnot be construed as limiting the scope of the invention in any way.

EXAMPLES Example 1 (2S, 3S, 4R,5R)-2-(5-tert-butyl-[1,3,4]oxadiazol-2-yl)-5-[6-(4-chloro-2-fluoro-phenylamino)-purin-9-yl]-tetrahydro-furan-3,4-diol

(a)

To a mixture of 40.0 g (95.1 mmol) of9-[(3aR,4R,6S,6aS)-6-(5-tert-butyl-1,3,4-oxadiazol-2-yl)-2,2-dimethyltetrahydrofuro[3,4-d][1,3]dioxol-4-yl]-6-chloro-9H-purineand 9.52 g (95.1 mmol) of CaCO₃ powder was successively added 200 mL ofglacial acetic acid and 21.1 mL (190 mmol) of 4-chloro-2-fluoroanilineat ambient temperature under nitrogen. The light brown mixture washeated at 76° C. for 1.5 to 2.0 h. The mixture was cooled to below 30°C. and diluted with 200 mL of toluene. The mixture was concentrated toabout 120 mL by distillation at 35-45° C. under vacuum below 50 torr.This azeotropic process was repeated with a second charge of 240 mL oftoluene and distilled to about 200 mL. The above mixture was filtered bysuction and the filtrate was concentrated to about 80 mL at 35-45° C.under vacuum to provide a toluene solution of crude product ofN-{9-[(3aR,6S,6aS)-6-(5-tert-butyl-1,3,4-oxadiazol-2-yl)-2,2-dimethyltetrahydrofuro[3,4-d][1,3]dioxol-4-yl]-9H-purin-6-yl}N-(4-chloro-2-fluorophenyl)amine.

(b)

To the above mixture was added 160 mL of 19:1 mixture of trifluoroaceticacid and water over 10 min with ice water cooling. Upon completion ofthe deacetonization after 22 h at 0-5° C., the reaction was quenched byaddition of 800 mL of 2.5 N NaOH over 15 min with ice-water cooling. Theresultant white solid was filtered. The filtering cake was washed with500 mL of water and briefly dried to give 62.3 g of crude productcontaining 4-chloro-2-fluoroaniline. Recrystallization from acetonitrileprovided 34.3 g (74%) of (2S, 3S, 4R, 5R)-2-(5tert-butyl-[1,3,4]oxadiazol-2-yl)-5-[6-(4-chloro-2-fluoro-phenylamino)-purin-9-yl]-tetrahydro-furan-3,4-diolas a crystalline white solid. ¹H NMR (300 MHz) δ 1.31 (s, 9H), 4.81 (m,1H), 4.99 (m, 1H), 5.17 (d, J=3 Hz, 1H), 5.92 (m, 1H), 6.16 (d, J=3 Hz,1H), 7.32 (m, 1H), 7.51 (m, 1H), 7.64 (m, 1H), 8.24 (m, 1H), 8.52 (m,1H), 9.74 (s, 1H).

Example 2 Comparative Data

The table below shows the results of a controlled study comparing theliterature method described in Fleysher, M. H. et al., J. Med. Chem.(1969) 12:1056 with the method of the present invention*.

*To a mixture of 300 mg (0.713 mmol) of9-[(3aR,4R,6S,6aS)-6-(5-tert-butyl-1,3,4-oxadiazol-2-yl)-2,2-dimethyltetrahydrofuro[3,4-[][1,3]dioxol-4-yl]-6-chloro-9H-purine and 0.24 mL (2.14 mmol) of4-chloro-2-fluoroaniline in 4.7 mL of absolute ethanol was successivelyadded 0-143 mg (0-1.43 mmol) of CaCO₃ powder and 0-35 μL (0-143 mmol) of4 N HCl in 1,4-dioxane at ambient temperature. The mixture was heated atreflux and aliquot samples were taken at various time of the reaction.The samples were analysed by the HPLC method as follows. Column: LunaC18 50×2 mm, 3 μm; wavelength: 270 nm; flow rate: 1 mL/min at 40° C.;mobile phase A: H₂O (0.05% TFA); mobile phase B: MeCN (0.05% TFA);gradient 0-95% B over 8 min. Peaks of the starting chloropurinenucleoside, the productN-{9-[(3aR,6S,6aS)-6-(5-tert-butyl-1,3,4-oxadiazol-2-yl)-2,2-dimethyltetrahydrofuro[3,4-d[1,3]dioxol-4-yl]-9H-purin-6-yl}-N-(4-chloro-2-fluorophenyl)and three major impurities derived from decomposition of the isoxazoleand acetonide moieties were examined. Results Starting* Major Product*Material Impurities* Reaction (AUC %) (AUC %) (AUC %) conditions 1.5 h3.0 h 1.5 h 3.0 h 1.5 h 3.0 h A Control 5.0 64 69 16 0  3.2 B 4N HCl(0.2 eq) 71 57 4.6 0 10 27 C CaCO₃ (2eq), 69 73 8.7 3.2 4.5  7.7 4N HCl(0.1 eq) D CaCO₃ (2eq) 17 72** 56 1.2** 0 11***The area under curve (AUC) readings for HPLC was not adjusted for4-chloro-2-fluoroaniline.**These results were based on HPLC analysis after reflux for 5 hours.Data for 3.0 h is not available.

Compared with the CaCO₃ method described in the literature, the methodof the present invention is faster. After 1.5 h, the control reaction(no CaCO₃, no HCl) had only 5% product, HCl reaction 71%, CaCO₃/HClreaction 69% and CaCO₃ reaction 17%. These numbers demonstrate thataddition of HCl to the reaction mixture increased reaction rate. CaCO₃appeared to reduce the level of impurities (compare impurity level forreactions B and C, 10% vs. 4.5% at the 1.5^(th) h and 27% vs. 7.7% atthe 3^(rd) h). For results at the 3 h, the last reaction (D) is not adirect comparison due to the fact the numbers in ** were taken at the5^(th) hour.

In addition to the above data, it should be noted that when acetic acidwas used as catalyst/solvent in the presence of only one equivalent ofCaCO₃, HPLC showed 87% product, 1.7% starting material and only 2.5%impurities after only 1.0 h at 76° C.

The controlled study not only confirmed the acceleration of thereaction, but also the reduction in the amount of by-product derivedfrom the acid induced degradation.

1-10. (canceled)
 11. A process for preparing N⁶-substituted aminopurineribofuranose nucleosides comprising the step of reacting a 6-halopurineribofuranose nucleoside with an amine in the presence of CaCO₃,characterised in that acid is added to the reaction mixture at the startof the reaction.
 12. The process according to claim 11 wherein aN⁶-substituted aminopurine ribofuranose nucleoside of formula (I):

is prepared by reacting a 6-halopurine ribofuranose nucleoside offormula (II)

with an amine of formula (III)H₂N—R¹  (III) in the presence of CaCO₃; wherein in formulae (I), (II),and (Ill): L is halogen; R¹ is hydrogen or a group selected from: (i)-(alk)_(n)-(C₃₋₉)cycloalkyl or -(alk)_(n)-(C₃₋₉)cycloalkenyl, saidcycloalkyl or cycloalkenyl group being optionally substituted by one ormore substituents selected from OH, halogen, C₁₋₆alkyl, —C₁₋₆alkoxy,C₂₋₆ alkenyloxy-, C₂₋₆ alkynyloxy-, and phenyl, wherein (alk) isC₁₋₃alkyl and n represents 0 or 1, and said (alk) group may beoptionally substituted by a C₃₋₆cycloalkyl group; (ii) a phenyl groupoptionally substituted by one or more substituents selected from:halogen, OH, CF₃, cyano, —C₁₋₆alkyl, —C₂₋₆alkenyl, —C₂₋₆alkynyl,C₁₋₆alkoxy-, —C₁₋₆alkylOH, —CO₂H, and —CO₂C₁₋₆ alkyl; (iii) aC₄₋₇aliphatic heterocyclic group containing at least one heteroatomselected from O, N or S, and optionally substituted by one or moresubstituents selected from: OH, —C₁₋₆alkyl, —C₁₋₆alkoxy,—CO₂(C₁₋₄)alkyl, —CO(C₁₋₄)alkyl, —CO₂aryl, and—CO₂(alk)_(n)(C₃₋₆)cycloalkyl, wherein (alk) is C₁₋₃alkyl and nrepresents 0 or 1; (iv) a straight or branched C₁₋₁₂alkyl groupoptionally substituted by one or more groups selected from phenyl,halogen, hydroxy, and C₃₋₇ cycloalkyl, wherein one or more carbon atomsof the C₁₋₁₂alkyl group may be optionally replaced by a groupindependently selected from S(═O)_(n) (where n is 0, 1 or 2) and N; (v)a fused bicyclic ring

wherein A represents C₄₋₆cycloalkyl or phenyl and B represents phenyloptionally substituted by C₁₋₃alkyl, and the bicyclic ring is attachedto the purine-6-amino moiety via a ring atom of ring A; R² is C₁₋₃alkyl,C₁₋₃alkenyl, C₁₋₃alkoxy, halogen or hydrogen; R³ is (i) hydrogen, (ii)C₁₋₆alkyl optionally substituted by one or more halogens, (iii) C₁₋₆alkylOCH₂— where the alkyl chain is optionally substituted by one ormore halogens, (iv) an acetylene group, or (v) a 5-membered heterocyclicgroup optionally substituted by: C₁₋₆alkoxy-, —C₁₋₆alkylO(CH₂)_(n)—where n is 0-6, C₃₋₇cycloalkyl, C₁₋₆hydroxyalkyl, halogen, a —C₁₋₆alkyl,—C₁₋₆alkenyl, or —C₁₋₆alkynyl group optionally substituted by one ormore halogens; R⁴ and R⁵ independently are hydrogen, acyl, —C₁₋₆alkyl ora suitable protecting group.
 13. The process according to claim 12wherein the 6-halopurine ribofuranose nucleoside is a 6-chloropurineribofuranose nucleoside.
 14. The process according to a claim 13 whereinthe acid is selected from the group consisting of aliphatic and aromaticcarboxylic acids, aliphatic and aromatic sulfonic acids, halogen acids,and mineral acids.
 15. The process according to claim 14 wherein theacid is acetic acid.
 16. The process according to claim 15 wherein thecompound of formula (I) is (2S, 3S, 4R,5R)-2-(5-tert-butyl-[1,3,4]oxadiazol-2-yl)-5-[6-(4-chloro-2-fluoro-phenylamino)-purin-9-yl]-tetrahydro-furan-3,4-diol.17. A compound prepared by the process according to claim
 11. 18. Acompound prepared by the process according to claim
 12. 19. A compoundprepared by the process according to claim 12, which compound is activeat one or more adenosine receptors.
 20. A compound prepared by theprocess according to claim 12, which compound is an agonist at theAdenosine A1 receptor.
 21. A compound prepared by the process accordingto claim 12, which compound is (2S, 3S, 4R,5R)-2-(5-tert-butyl-[1,3,4]oxadiazol-2-yl)-5-[6-(4-chloro-2-fluoro-phenylamino)-purin-9-yl}-tertahydro-furan-6,4-diol.